Add ui-shared.h

This is finally a proper headerfile for the shared ui-functions which
used to reside in cgit.h

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-diff.c

@@ -8,6 +8,7 @@
 
 #include "cgit.h"
 #include "html.h"
+#include "ui-shared.h"
 
 unsigned char old_rev_sha1[20];
 unsigned char new_rev_sha1[20];